In recent years, optical network systems for realizing communication speeds of 40 Gbps have been actively introduced in order to cope with increasing communication traffic. In an optical transmission apparatus for sending and receiving optical signals within an optical network system, dispersion compensation is provided by a VDC (variable dispersion compensator) to compensate for distortion (dispersion) of waveforms produced in optical signal pulses as the communication speed is increased.
For example, JP-A-2002-208892 or JP-A-2003-224523 discloses a technique for holding the wavelength dispersion value of a VDC either at a position where the bit error rate (BER) is lowest (for example, see (a) of FIGS. 10A and 10B) or at a given position where the bit error rate is lower than a prescribed BER value (for example, see (b) of FIGS. 10A and 10B) in a parabola indicating the dispersion characteristics for transmitted optical signals.
Furthermore, the above-cited JP-A-2002-208892 discloses a technique which, when the lowest point of the bit error rate is not found, sets an optimum wavelength dispersion value at the midpoint (for example, see (e) of FIGS. 10A and 10B) between the wavelength dispersion values of the variable dispersion compensator at two points (for example, see (c) and (d) of FIGS. 10A and 10B) at which the parabola indicating the dispersion characteristics for transmitted optical signals intersects with a level line of a preset threshold value for the BER.
Furthermore, the above-cited JP-A-2002-208892 discloses a technique of creating a threshold value for a reset operation to trigger the reset operation in order to reset an optimum wavelength dispersion value according to variation of the dispersion characteristics for an optical signal caused, for example, by outside air temperature variations.